The present invention relates to a brake cylinder device having a service piston activated by fluid under pressure and a spring-applied parking brake piston (for parking and/or emergency braking) held in an unlocked position by the pressure of the fluid acting against the reaction force of the spring.
Spring-applied brakes are well known, especially braking devices having compressed air service brakes combined with spring brakes held released by compressed air pressure during operation of the vehicle. In addition to the risks of sudden application of spring brakes in case of rupture of the line feeding the spring brake piston, the principal drawback of these spring brakes, that otherwise present excellent compatibility with pneumatic service braking, rests in the fact that the reaction force of the spring that applies the parking or emergency brake rapidly diminishes as the spring expands. Inevitably, significant slack exists in the rigging due to flexure of the parts, as well as brake shoe-wheel wear, thus increasing the stroke of the parking brake piston and, consequently, the degree of expansion of the parking brake spring. It has been noted as well that, in numerous cases, after several years of spring brake service, the spring reaction force is diminished by the effect of corrosion and the creep of spring steel, when remaining under constraint for a period of time, so that, in braking position, only values between one-half and one-third of the initial maximum reaction force are realized.